Wednesday, July 14, 2010

Since the first exoplanet — a planet outside our solar system — was discovered in 1995, more than 460 others have been found. While astronomers have been able to measure the size, orbital characteristics, and even some of the molecules that make up the atmospheres of some exoplanets, many mysteries about their formation and evolution remain.

A team of astronomers, including a researcher from MIT’s Kavli Institute for Astrophysics and Space Research, has become the first to measure wind in the atmosphere of an exoplanet. By detecting heavy winds on HD209458b, a huge exoplanet located 150 light years away that is slightly more than half the mass of Jupiter, the researchers could then measure the movement of the planet as it orbited its host star — also another first for exoplanetary research.

The work, which is detailed in a paper published June 24 in Nature, will guide future research on exoplanets, since understanding the properties of a planet’s atmosphere is a critical first step for characterizing how that planet formed and evolved.

Measuring the planet’s orbital movement is also important because the velocity of that movement can be used with Newton’s law of universal gravitation to get a more precise estimate of the mass of both the planet and its parent star. Before now, astronomers had to rely on complex mathematical models, as well as the changes in light that occurred when an exoplanet’s host star wobbled in response to the exoplanet’s gravitational pull, to determine the exoplanet’s mass. Thanks to a new technique that the researchers used to study HD209458b, astronomers should now be able to refine their estimates of the mass of some exoplanets and their stars.

One way that astronomers can learn a lot about an exoplanet is by observing it as it passes in front of its host star as seen from Earth. By measuring the light obscured by an exoplanet during this event, which is known as a transit, astronomers can learn details about the planet, such as its size and what kinds of molecules exist in its atmosphere. Of the 463 exoplanets discovered to date, more than 80 are known to be transiting planets. (HD209458b, identified in 1999, was the first transiting exoplanet discovered.)

Researchers detected the heavy winds in HD209458b’s atmosphere by studying carbon monoxide. According to co-author and Kavli postdoc Simon Albrecht, who collaborated with researchers from Leiden University and the Netherlands Institute for Space Research (SRON), the results are “among the many small steps the astronomy community is taking toward being able to, at some point, measure atmospheric conditions on exoplanets that are twins to our Earth.”

What makes the work, partly funded by the Netherlands Organisation for Scientific Research, “potentially groundbreaking” is the ground-based technique that was used to detect the winds and orbital movement of HD209458b, according to Adam Showman, a planetary scientist at the University of Arizona. “Just the fact this is even doable from the ground is spectacular,” he said.

Instead of using a space-based instrument like NASA’s Spitzer Space Telescope to study the faraway planet, the researchers used a ground-based, high-resolution spectrograph at the European Southern Observatory in Chile that can detect subtle changes in the wavelength of light when a planet transits its star. As HD209458b transited last August, its parent star left what lead author Ignas Snellen from the Leiden Observatory in the Netherlands described as “a fingerprint” of light that filtered through the planet’s atmosphere. The researchers then used the spectrograph to analyze that imprint of light to detect carbon monoxide molecules in the atmosphere. “It seems that H209458b is actually as carbon-rich as Jupiter and Saturn, and this could indicate that it was formed in the same way,” Snellen said.

The researchers then spent several months analyzing spectrographic measurements of the movement of the carbon monoxide thanks to the Doppler shift, a phenomenon that creates subtle color changes in wavelengths of light when something moves. When an object moves toward us, it looks slightly bluer, and when it moves away, it looks slightly redder. The spectrograph revealed color shifts in the light absorbed by the exoplanet, which indicated that something was moving the gas. That something, the researchers believe, is heavy wind that is blowing carbon monoxide in the planet’s atmosphere up to 10,000 kilometers per hour (the fastest winds ever detected on another planet in our solar system were blowing at up to 2,000 kilometers per hour on Neptune, according to previous research). By tracking the movement of the carbon monoxide, the astronomers could then measure the movement of the planet as it orbited its host star.

While the results are notable, future research must address what might be causing the heavy winds, said Showman. Right now, the spectrograph simply does not have enough spectral resolution to distinguish that level of detail.

As the team continues to refine the ground-based technique used in this research, Albrecht said that he and his colleagues must do “a better job” of analyzing exoplanetary atmospheres for molecules that have fainter spectral signals than carbon monoxide, such as water. Their next step is to measure the atmospheres of exoplanets that are located slightly farther away from their host stars to see how this distance affects detectable concentrations of carbon monoxide and other molecules.